Grafting of high and low molecular weight polymers has been reported often in the literature [see for example, polymer textbook "Polymer Chemistry" by M. P. Stevens, (Addison-Wesley), 1975, pp 196-202]. Maleation is a subgroup of grafting. Those skilled in the art recognize the highly individualistic nature of the maleation processes which have been patented. For example, maleation of polypropylenes progresses easily to higher acid numbers with free radical initiation (see U.S. Pat. Nos. 3,414,551, 3,480,580, 3,481,910, 3,642,722, 3,746,676, 3,932,368, and 4,613,679). The molecular weight of the products of such reactions are lower than that of the starting polypropylene due to accompanying degradation reactions. On the other hand direct maleation of polyethylenes results in cross-linking which increases the molecular weight of the polymer [see for example, "Journal of Applied Polymer Science" 44, 1941, N. G. Gaylord et al (1992); and U.S. Pat. Nos. 4,026,967, 4,028,436, 4,031,062, 4,071,494, 4,218,263, 4,315,863, 4,347,341, 4,358,564, 4,376,855, 4,506,056, 4,632,962, 4,780,228, 4,987,190, and 5,021,510]. Free radical initiated maleation of polyethylenes in continuous processes is limited to very low acid numbers in order to prevent gelation due to excessive crosslinking. Continuous processes provide lower cost commercial products. This means, that heretofore commercial maleated polyethylenes have been lower acid number since the maleation to high acid number in a continuous production unit forms high molecular weight gels that clog the reactor. Thermal maleations without the use of initiators is also employed for maleation of polyethylenes in order to minimize crosslinking. The lower acid numbered (10 or below) maleated polyethylenes which are offered commercially are prepared from molten polyethylene by a continuous thermal maleation process (no peroxide) at temperatures near or above 300.degree. C. Free radical initiated maleation of polyethylenes to a high acid number using special reagents have been attempted in a batchwise process. However, the use of these special reagents produce a product that is high in color and/or gives off vapors offensive to humans handling the processing and use of this maleated product.
Different techniques of maleation yield different product types. Solid state maleations, those carried out below the melting point of the polymer, occur on the exposed surface of the solid. The acid numbers attained are necessarily a function of exposed surface area. Solvent based processes dissolve the polymer and produce a much more uniform maleated product [see U.S. Pat. Nos. 3,416,990, 3,437,550, 3,483,276, 3,928,687, 4,078,017, 4,299,754, 4,624,992, and 4,693,838 and Japanese Patents 59 105,053, 84 105,053 (1984), 69 15,422 (1969), 69 15,423 (1969), 77 93,495 (1977), 80 34,224 (1980), and 82 42,736 (1982). Solvent removal and recycling is an added expense of such processes however. An extruder serves as the reactor in some processes and provides some decrease in the molecular weight of the polymer due to mechanical tearing of the polymer chains [see U.S. Pat. Nos. 3,862,265, 4,003,874, 4,548,993, 4,639,495, 4,751,270, 4,762,890, 4,857,600, 4,927,888, and 5,001,197 and Japanese Patents JP 63,309,540, 88,309,540 (1988) and JP 78,137,292 (1978) and European Patent Application EP 280454 (Aug. 31, 1988)]. Some other processes resort to other chemical reactions such as oxidation or reaction with an alcohol or amine, either before maleation or after maleation, to provide unique products [see U.S. Pat. Nos. 4,443,584 and 4,727,120 and Japanese Patent JP 77 08,035 (1975)]. In other processes copolymers are used to alter the change in molecular weight i.e. the increases in molecular weight, (maleation of polyethylenes), or the decreases in molecular weight (maleation of polypropylenes) (see U.S. Pat. Nos. 3,953,541, 4,533,700, 4,612,155, 4,749,505, and 4,822,688). Finally, the processes which are carried out in water must involve maleation with a mixture of maleic acid and maleic anhydride at best, and possibly involve maleic acid exclusively (see U.S. Pat. Nos. 4,370,450, 4,839,423, 4,877,841, and 4,879,347). The product of these aqueous processes are necessarily the carboxylic acids rather than the anhydrides which are obtained in other maleation reactions.
The maleated polyethylene waxes of high acid number would be very desirable in that higher acid numbers increase the emulsifiability of the waxes; the scuff and black heel mark resistance of floor polishes containing the waxes; and the adhesion of the waxes making them more useful as compatibilizing agents and surface coatings. However, attempts to produce high acid number polyethylene waxes that have essentially no color or objectionable odor have not been successful. Additionally, the commercialization of batchwise production of any high acid number polyethylenes has been impractical. Thus, it would be very desirable to be able to produce a useful high acid number maleated polyethylene. More particularly, it would be even more desirable and inexpensive if these were conducted in a continuous process adaptable to commercial production.